This is a continuation-in-part of pending U.S. patent application Ser. No. 07/553,073, filed Jul. 10, 1990.
This invention relates to nuclear reactors and, more particularly, to fuel arrangements in a reactor core. A major objective of the present invention is to provide for more thorough fuel burnups to enhance fuel utilization and minimize active waste products.
Dual-phase fission reactors store heat generated by a reactor core primarily in the form a phase conversion of a heat transfer medium from a liquid phase to a vapor phase. The vapor phase can used to physically transfer stored energy to a turbine that drives a generator, to produce electricity. Condensate from the turbine can be returned to the reactor, merging with recirculating liquid for further heat transfer and cooling. The primary example of a dual-phase reactor is a boiling-water reactor (BWR). Dual-phase reactors are contrasted with single-phase reactors, which store energy primarily in the form of elevated temperatures of a liquid heat-transfer medium, such as liquid metal. The following discussion relating to BWRs is readily generalizable to other forms of dual-phase reactors.
Fission reactors rely on fissioning of fissile atoms such as uranium isotopes (U233, U235) and plutonium isotopes (Pu239, Pu241). Upon absorption of a neutron, a fissile atom can disintegrate, yielding atoms of lower atomic weight and high kinetic energy along with several high-energy neutrons. The kinetic energy of the fission products is quickly dissipated as heat, which is the primary energy product of nuclear reactors. Some of the neutrons released during disintegration can be absorbed by other fissile atoms, causing a chain reaction of disintegration and heat generation. The fissile atoms in nuclear reactors are arranged so that the chain reaction can be self-sustaining.
The ability of neutrons to contribute to fissioning depends on the quantity (flux) of neutrons within a "thermal" neutron spectrum. Initially, neutrons released during fissioning move too quickly and have too high an energy to readily induce the further fissioning required to sustain a chain reaction. These high energy neutrons are known as "fast" neutrons. Slower neutrons, referred to as "thermal neutrons", most readily induce fission.
In BWRs, thermal neutrons are formerly fast neutrons that have been slowed primarily through collisions with hydrogen atoms in the water used as the heat transfer medium. Between the energy levels of thermal and fast neutrons are "epi-thermal" neutrons. Epithermal neutrons exceed the desired energy for inducing fission but promote resonance absorption by many actinide series isotopes, converting some "fertile" isotopes to "fissile" (fissionable) isotopes. For example, epithermal neutrons are effective at converting fertile U238 to fissile Pu239.
To facilitate handling, fissile fuel is typically maintained in modular units. These units can be bundles of vertically extending fuel rods. Each rod has a cladding which encloses a stack of fissile fuel pellets. The bundles are arranged in a two-dimensional array in the reactor. Neutron-absorbing control rods are inserted between or within fuel bundles to control the reactivity of the core. The reactivity of the core can be adjusted by incremental insertions and withdrawals of the control rods.
Both economic and safety considerations favor longer fuel bundle lifetimes and less frequent refuelings. For a given bundle design, the most obvious method of increasing fuel bundle lifetime is to incorporate more fissile fuel. This can be accomplished without increasing fuel volume by using enriched fissile fuel. However, concerns over criticality limit enrichment to about 5%.
Given optimization of fuel bundle size and fuel enrichment, enhancing fuel bundle lifetimes requires improved fuel utilization, which in turn generally implies more complete fuel "burnups". Approaching complete fuel burnups is made more difficult by temporal variations in the fissioning process. Fissioning continually changes the composition of a fuel bundle, and thus its reactivity. Generally, the changes result in a net decline in reactivity so that a fuel bundle tends to burn hotter near the beginning of its life and cooler near the end.
To some extent the effects of changing composition on reactivity can be addressed by moving the control rods. By inserting control rods early in a bundle lifetime initial burnup rates can be limited. As fissile fuel is spent, the control rods can be withdrawn gradually to even out the power production from a fuel bundle over time. However, the effect of control rod movement on reactivity is too gross to maximize fuel bundle lifetimes.
Burnable poisons, such as gadolinium oxide (Gd.sub.2 O.sub.3), can be included in a fuel bundle to limit its reactivity early in its lifetime. Burnable poisons compete with fissile fuel for thermal neutrons, limiting their availability for fissioning. Over time, as they absorb neutrons, the burnable poisons are converted to nonpoisonous isotopes so that more thermal neutrons become available for fissioning. The decrease in burnable poisons counters the loss in reactivity that occurs due to the decrease in fissile fuel due to fissioning. In effect, some of the would-be early fissioning is postponed to later in the fuel bundle lifetime when it can be more efficiently utilized. However, burnable poisons leave a poison residue, causing a bundle to expire while retaining a larger quantity of fissile material than would be retained without the poison. This early expiration not only adversely affects fuel efficiency, but also imposes an increased burden on waste disposal.
What is needed is an approach for extending fuel bundle lifetime without requiring bundle resizing or unacceptably high levels of enrichment. Preferably, the approach should allow more complete fuel burnups to minimize the waste disposal problems associated with fuel bundles using burnable poisons.